In ink jet printing, images are produced from ink droplets ejected from nozzles in the print head and deposited on to a substrate. In order to accurately reproduce the image required, it is necessary to have close control over both the size of the ink droplets ejected and the direction in which they travel after detachment from the plate. Ink puddles near the ejecting nozzles in ink jet printing devices, both thermal and piezo driven, can adversely affect the trajectory of the ejected droplets, resulting in poor print quality. Interaction between the print head surface and the ink droplet has therefore to be closely controlled in order to maintain clean breakaway of the droplets. Generally speaking, to control the phenomena of ink puddling and to avoid the mixing of different inks, orifice plate surfaces with high hydrophobicity are preferred.
A range of different methods and materials has been employed by the industry to modify the surface properties of orifice plates, in order to obtain satisfactory print quality. The materials used depend, amongst other things, on the material of construction of the orifice plate and the type of printer it is being used on.
One possible solution to the problem is to apply a layer of fluorocarbon coating to the surface of the plate. However, though such materials provide excellent anti-wetting properties (which can be judged from a high contact angle water forms with the coated surface) they do pose other problems. It is generally difficult to get the fluorinated material to bind effectively to the plate surface, thus to ensure good adhesion of the layer, an intermediate coating layer is generally required. Such a two-layer process adds significantly to processing times and costs.
One such technology, described in U.S. Pat. Nos. 6,283,578 and 6,312,085, employs a siloxane polymer layer, formed from a mixture of silane precursors as the adhesion promoting layer onto which is deposited a monolayer coating of a perfluoroalkyltrialkoxysilane. However, the use of dual layer coating processes is time consuming and generally not cost efficient.
In U.S. Pat. No. 5,910,372 polysiloxane coatings are also employed. Several silane precursor types are mixed to give a single layer coating that combines the benefits of the two layer coatings described in U.S. Pat. No. 6,283,578. The coatings contain low levels of two different functional silanes, the bulk of the coating being composed of a non-functional silane. Amine functional silanes are included, which bind to the substrate and perfluoroalkyl silanes that migrate to the coating surface to give a low surface energy exterior. However, this technology has several limitations. It seems to be preferred for use on surfaces such as polyimide, to which the amines bind well. The coating process also involves several time consuming steps. After application, the coating is left to stand for five minute to allow phase separation of the different components in the coating to occur. Coatings are then cured for three hours at 95° C. under conditions of high humidity. The coatings show good resistance to ink, but are degraded by wiping which wears away the top surface in which the hydrophobic functionality is concentrated.
In addition, the use of different functional molecules with hydrophobic tails for monolayer coatings of print heads has also been proposed. The functional group of the respective molecule attaches to the plate surface of the print head, while the hydrophobic tail results in a low surface energy coating. Such monolayers of perfluoropolyether chain containing alkoxysilanes are claimed to be effective in EP patent application 1,273,448 A1. U.S. patent application 2002/0097297 A1 and U.S. Pat. No. 6,325,490 report monolayer coatings of alkyl thiols, while U.S. Pat. Nos. 6,151,045 and 6,345,880 describe the use of functionalised polydimethylsiloxane oligomers in such monolayers.
However, the practical application of such monolayers in ink jet printers may be problematic. Once ink accumulates on the orifice plate surface, the plate is wiped periodically with a wiper blade to clean the plate surface. Monolayer coatings as described above may not have sufficient durability to withstand this wiping action during a long life time that may thus result in damage to the coating and a change in the ink wetting properties of the surface. This in turn would lead to a decrease in print quality.
Accordingly, there remains the need for coating materials that adhere well to a surface of a print head, such as an orifice plate surface, and that is wear resistant so that it is not degraded by the wiping process used to clean ink from the orifice plate. The coating should also show high water contact angle and ink-contact angles that are not degraded by long-term exposure to ink.